It is generally well known that magnetic survey tools are disturbed in varying ways by anomalous magnetic fields associated with fixed or induced magnetic fields in elements of the drill string. It is further well known that the predominant error component lies along the axis of the drill string. This latter fact is the basis for several patented procedures, to eliminate the along-axis field errors in 3-magnetometer survey tools. Among these are U.S. Pat. Nos:
4,163,324 to Russell et al. PA0 4,433,491 to Ott et al. PA0 4,510,696 to Roeslet PA0 4,709,486 to Walters PA0 4,682,421 to Van Dongen et al. PA0 4,761,889 to Cobern et al. PA0 4,819,336 to Russel PA0 5,155,916 to Engebretson PA0 U.K. patents 2,138,141A to Russell et al. and 2,185,580 to Russell.
Engebretson U.S. Pat. No. 5,155,916 provides a method for error reduction in compensation for magnetic interference.
All of these methods, in effect, ignore the output of the along-axis magnetometer, except perhaps for selecting a sign for a square root computation. They provide an azimuth result by computation of a synthetic solution, either:
1) by using only the two cross-axis magnetometers and known characteristics of the Earth field, or
2) by using the cross-axis components and an along-axis component computed from the cross-axis components and known characteristics of the Earth's field.
Most of these require, as the known characteristics of the Earth field, one or more of the following:
1) field magnitude
2) dip angle
3) horizontal component
4) vertical component.
The Walters method requires, as known characteristics of the Earth field, only that:
1) the field magnitude is constant in the survey area;
2) the dip angle is constant in the survey area.
The fact that these quantities are constant is all that is required. The value of the constant is not needed but is derived within the correction algorithm.
Since all of these compensation methods use, in effect, a computed along-axis component, all of them break down for cases of borehole high inclination angles in a generally East/West direction. This is because the cross-axis measurement plane for such condition tends to be aligned so as to contain both the gravity and Earth field vectors, and thus measurements in this plane provide a poor measure of the cross product of the Earth field and gravity vectors. The cross product vector of the two reference vectors is the vector that actually contains the directional reference information.
The actual degradation of accuracy at high inclinations in the East/West direction for the previously cited methods depends both on the inherent accuracy of the sensors in the survey tool and on the accuracy of the required knowledge of the Earth field characteristics.
To provide a mechanization for a magnetometer survey tool that does not seriously degrade in accuracy at borehole higher inclination angles near the East/West direction, it is found to be necessary to provide a method and means to calibrate the errors in the along-axis magnetometer so that accurate measurements can be made with it. This is in direct contrast with existing methods that substitute computed values for along-axis measurements.
There is, therefore, need to provide a calibration method for an along-axis magnetometer in a magnetic survey tool to correct anomalous magnetic effects in a drill string and thereby to permit accurate measurements of the along-axis component of the Earth magnetic field. Such accurate measurement of the along-axis component then permits accurate computation of azimuthal direction independent of inclination and direction.